This invention relates to the rapid and efficient recovery of dissolved molybdenum from a solution of mixed sulfuric acid-nitric acid.
The processing of tungsten wire into filament coils, especially double-coiled filaments, requires the use of molybdenum mandrels around which the coil is wrapped. After the tungsten wire is thus formed and cut to size and hydrogen baked to permanently set the coil turns, the entire member is immersed in a warm solution of mixed sulfuric acid and nitric acid which dissolves the retained molybdenum mandrel leaving the formed tungsten coil intact. The resulting coil of tungsten is then in a form suitable for use, such as a filament for an electric lamp.
The relative concentrations of the mixed sulfuric acid and nitric acid are subject to some variation and a mixed acid solution of sulfuric acid (normality of thirteen) and nitric acid (normality of seven) has been found to be suitable. When this acid solution has sufficient molybdenum dissolved therein so that the molybdenum is present in an amount of from about 40 grams to about 75 grams per liter of acid solution, the rate of molybdenum dissolution becomes sufficiently slow that the acid is regarded as spent.
In U.S. Pat. No. 3,963,823, dated June 15, 1976, is disclosed a method for recovering the molybdenum from the spent acid. In accordance with the teachings of this patent, ammonium hydroxide is added to a diluted spent acid solution to partially neutralize same and raise the pH thereof to a value of from about 1.5 to about 3. The solution is then maintained in a heated condition for a predetermined time sufficient to precipitate therefrom substantially all of the dissolved molybdenum as ammonium molybdate which is then recovered for further use. More specifically, after initial partial neutralization, the solution temperature preferably is maintained at from 70.degree. C. to 80.degree. C., with constant solution agitation for a period of at least about ten hours and preferably from ten hours to twenty hours in order to precipitate the dissolved molybdenum as ammonium molybdate complex.
In U.S. Pat. No. 4,307,065 dated Dec. 22, 1981, there is provided a method for quickly recovering molybdenum in a form substantially free from other metals from a mixed sulfuric acid-nitric acid aqueous solution which has substantial quantities of molybdenum dissolved therein and which has been used to dissolve molybdenum mandrels from coiled tungsten filaments. In accordance with this improved method, the spent mixed acid solution is diluted with water to a predetermined acid-water volume ratio. There is then added to the dilute solution anhydrous ammonia at a predetermined rate with the acid-water ratio and the rate of addition of anhydrous ammonia causing the solution temperature to rise to at least about 90.degree. C. The solution is maintained at less than its boiling temperature and the anhydrous ammonia addition thereto is continued until the pH is from 1.5 to about 3.5. The partially neutralized solution is then seeded with a predetermined amount of ammonium molybdate solid particles and the seeded and heated solution is then maintained at a temperature of at least about 90.degree. C. but less than the solution boiling temperature for a period of from about one hour to about four hours. This shortened time is sufficient to precipitate substantially all molybdenum therefrom as ammonium molybdate. Thereafter, the resulting precipitated ammonium molybdate is separated from the residual solution.
In the commercial practice of U.S. Pat. No. 4,307,065 it is the usual practice to wash the precipitated ammonium molybdate one or more times with water after the precipitated crystals have been filtered from the spent acid solution. Washing is necessary because the remaining acid solution contains considerable amounts of dissolved salts such as ammonium nitrate and ammonium sulfate which will encrust on the ammonium molybdate crystals if the retained solution is allowed to dry before washing.
To save time in the washing, settling, and subsequent removal of the wash liquid from the ammonium molybdate crystals, the process of double-batching is employed wherein crystallized ammonium molybdate is allowed to settle and the neutralized acid solution is removed, but the precipitate is not discharged to the filter. Rather, additional dilute spent acid solution is added to the precipitate and the molybdenum is precipitated by the above method. In this way a larger quantity of precipitate is processed at each washing.
Often, however, the second precipitate is extremely fine and slow settling notwithstanding the first precipitate having had been coarse. These fine crystals require several hours for the second precipitate and washing slurries to adequately settle, offsetting the time savings associated with double-batching. The fine precipitate also causes the liquid to become and remain highly opaque even after essentially all crystals have adequately settled, rendering ineffective judgements on when and to what level to remove the liquid.